1. Field of the Invention
The invention relates to a microwave oscillator with dielectric resonator.
2. Description of the Prior Art
Microwave oscillators with a dielectric resonator are routinely used in the electronics art. They are basic components of many circuits, in particular in the field of television or telecommunications.
An oscillator of this kind has an active part with an amplifier or negative impedance and a passive part including the dielectric resonator.
The active part and the passive part can be coupled by reflection or by transmission.
FIG. 1 shows a prior art set-up using transmission.
In this set-up, the active component is an amplifier 10 and the dielectric resonator 12 is in series in a feedback loop connecting the output 11 of the amplifier 10 to its input 13.
The line 14 connected to the output 11 of the amplifier 10 is magnetically coupled to the resonator 12. Similarly, the input line 16 is magnetically coupled to the resonator 12. The coupling between the resonator 12 and each of the lines 14, 16 therefore increases as the current in the line increases. This is why a line 18 of length .lambda./4 is generally provided at the input to provide a short circuit and thereby maximize the current. .lambda. is the wavelength corresponding to the frequency of the oscillator.
An oscillator of the above kind must conventionally satisfy two conditions, namely, on the one hand, the condition that the gain of the amplifier 10 must be greater than the losses of the feedback circuit between the output 11 and the input 13 and, on the other hand, the condition that there must be zero phase shift (modulo 2.pi.) of the signals across the circuit. To satisfy the second condition, the points at which the conductors 14 and 16 are coupled to the resonator 12 are diametrally opposed relative to the disk-shaped resonator, for example. Under these conditions, the resonator introduces a phase shift of .pi. radians and the amplifier 10 and the associated lines introduce a complementary phase shift of .pi. radians.
This type of oscillator has the advantage of being highly reproducible: it therefore lends itself to mass production as it requires few adjustments. On the other hand, it is not always easy to design because two couplings have to be determined: the couplings between the resonator and the lines 14 and 16. Furthermore, as it is necessary to provide a line 18 whose length is a function of the wavelength in order to produce the short circuit, the bandwidth of the associated circuit is small.
FIG. 2 shows a reflection type dielectric resonator oscillator which has two active components 20 and 22 and a dielectric resonator 24.
Each active component is coupled to the resonator 24 by reflection. The resonator reflects a wave emitted by an active component back to the active component, which amplifies it. For oscillation to be obtained it is also necessary to satisfy two conditions: the gain of the active component must be greater than the losses and the reflected waves must be in phase with the emitted waves. The second condition is satisfied by adjusting the distance between the coupling plane of the resonator and the ports of the active components.
In the so-called "push-push" set-up shown in FIG. 2, the oscillations produced are in antiphase because the point 23 at which the active component 22 is coupled to the disk-shaped resonator 24 is diametrally opposite the point 21 at which the active component 20 is coupled to the resonator 24. These waves in antiphase are transferred to an output point 32 by a combiner with lines 28 and 30.
In a set-up of this kind the resonator 24 is between two microstrip lines 23.sub.1 and 23.sub.2. The function of the resonator is to enable oscillation and to maintain the signals of the two oscillators in antiphase. It has been found that the oscillations are not maintained in antiphase correctly, which degrades performance, if oscillation does not occur at exactly the resonant frequency of the resonator. In fact, the oscillator is intended to supply a frequency 2f.sub.0 which is twice the fundamental frequency f.sub.0 by exploiting the fact that the output waves are in antiphase at the frequency f.sub.0 but cannot be in phase at double the frequency. The combiner with lines 28 and 30 eliminates the waves which are in antiphase and adds the waves which are in phase. However, because the characteristics of the combiner necessarily depend on the frequency f.sub.0, it is clear that the fundamental frequency will not be completely eliminated if oscillation occurs at a different frequency.
Note also that the resonator 24, having to be exactly symmetrical relative to the lines 23.sub.1 and 23.sub.2, is not easy to position, which is a particularly serious problem, especially in mass production. Furthermore, it requires manual adjustment, in particular of the impedance 34 connecting an electrode of the active component 20 or 22 to ground. However, the advantage of this set-up is its low phase noise.
The object of the invention is to provide a microwave oscillator which delivers a wave at the frequency 2f.sub.0 using a resonator which resonates at the frequency f.sub.0, which is easy to manufacture and whose operating parameters are relatively insensitive to adjustments.
The oscillator according to the invention includes at least two active components and a dielectric resonator and the coupling between each active component and the dielectric resonator is of the transmission type, the set-up being such that the input and the output of each active component are in antiphase at the resonant frequency of the resonator and the inputs of neighboring active components are connected to a first point coupled to the resonator and likewise the outputs of the active components are connected to a second point coupled to the resonator, the coupling of the ports to the resonator being such that they are practically short circuited.
In other words, it is the simultaneous presence of the active components that enables oscillation and the oscillations of two neighboring oscillators are synchronized with a phase difference of 180.degree.. Accordingly, compared to the "push-push" structure shown in FIG. 2, the resonator enables one or other of the basic oscillators to oscillate but is not required to maintain a phase difference of 180.degree. between the signals from the two oscillators. In other words, in the oscillator according to the invention, the two basic oscillators are synchronized independently of the resonator.
This makes the oscillator less sensitive to the parameters of the resonator.
Because the resonator of the oscillator according to the invention does not have to provide the phase difference of 180.degree. between the oscillations produced by each of the basic oscillators, it is not necessary for the resonator to be exactly symmetrical relative to the first and second ports, whereas in the "push-push" circuit oscillator shown in FIG. 2 the resonator 24 must be symmetrical relative to the lines 23.sub.1 and 23.sub.2. The invention therefore enables different decouplings between the resonator and, on the one hand, the first port and, on the other hand, the second port.
Note also that, to obtain the antiphase relationship between the waves supplied by each of the active components, it is not necessary to provide a combiner hose line lengths depend on .lambda., as is the case in the "push-push" set-up of FIG. 2. he associated circuit can therefore have a greater bandwidth than prior art set-ups like those shown in FIGS. 1 and 2.
Furthermore, note that, compared to an oscillator having a single active component and coupled to a dielectric resonator by transmission (FIG. 1), the short circuit is obtained without it being necessary to provide a line having a length of a quarter-wavelength.
Over and above this, compared to the "push-push" set-up, the fact that a combiner is not necessary simplifies designing the oscillator.